Single-wall carbon nanotubes, also commonly known as “buckytubes,” have unique properties, including high strength, stiffness, thermal and electrical conductivity. SWNTs are hollow, tubular fullerene molecules consisting essentially of sp2-hybridized carbon atoms typically arranged in hexagons and pentagons. SWNTs typically have diameters in the range of about 0.5 nanometers (nm) and about 3.5 nm, and lengths usually greater than about 50 nm. Background information on SWNTs can be found in B. I. Yakobson and R. E. Smalley, “Fullerene Nanotubes: C1,000,000 and Beyond,” American Scientist, 85:324-337 (1997) and Dresselhaus et al., Science of Fullerenes and Carbon Nanotubes, San Diego: Academic Press, Ch. 19, (1996) (hereinafter referred to as “Dresselhaus”).
The diameter and conformation of SWNTs can be described using the system of nomenclature described by Dresselhaus. Single-wall tubular fullerenes are distinguished from each other by a double index (n, m), where n and m are integers that describe how to cut a single strip of hexagonal graphite such that its edges join seamlessly when the strip is wrapped onto the surface of a cylinder. When the resultant tube is said to be of the “armchair” or (n, n) type, since when the tube is cut perpendicularly to the tube axis, only the sides of the hexagons are exposed and their pattern around the periphery of the tube edge resembles the arm and seat of an armchair repeated n times. When m=0, the resultant tube is said to be of the “zig-zag” or (n, 0) type, since when the tube is cut perpendicular to the tube axis, the edge is a zig-zag pattern. Where n≠m and m≠0, the resulting tube has chirality and contains a helical twist to it, the extent of which is dependent upon the chiral angle.
The electronic properties of SWNTs are dependent on the conformation. For example, armchair tubes are metallic and have extremely high electrical conductivity. All single-wall carbon nanotubes can be categorized as metallic, semi-metals, or semiconducting depending on their conformation. For clarity and conciseness, both metallic tubes and semi-metal tubes will be referred to collectively as metallic nanotubes. For single-wall carbon nanotubes, about one-third of the tubes are metallic and about two-thirds are semiconducting. Metallic (n, m)-type nanotubes are those that satisfy the condition: 2n+m=3q, or n−m=3q where “q” is an integer. Metallic nanotubes are conducting with a zero band gap in energy states. Nanotubes not satisfying either condition are semiconducting and have an energy band gap. Generally, semiconducting nanotubes with smaller diameters have larger energy band gaps. Regardless of tube type, all SWNTs have extremely high thermal conductivity and tensile strength.
The particular nanotube diameter and conformation affects the physical and electronic properties of the single-wall carbon nanotube. For example, the strength, stiffness, density, crystallinity, thermal conductivity, electrical conductivity, absorption, magnetic properties, response to doping, utility as semiconductors, optical properties such as absorption and luminescence, utility as emitters and detectors, energy transfer, heat conduction, reaction to changes in pH, buffering capacity, sensitivity to a range of chemicals, contraction and expansion by electrical charge or chemical interaction, nanoporous filtration membranes and many more properties are affected by the diameter and conformation of the single-wall carbon nanotube.
From an electronics perspective, separation of SWNTs according to type (metallic versus semiconducting) may be critical for certain applications such as the construction of quantum wires, while separation by diameter for semiconducting SWNTs may be of paramount importance in the microelectronics or optical arena (e.g., because diameter governs their band-gap).
One recent approach which permits the selective preservation of the semiconducting types of nanotubes in bundles, or “ropes,” of aligned single-walled carbon nanotubes has been demonstrated by IBM Corp. In that method, ropes of nanotubes of random chiralities are deposited on a silicon wafer that is then covered by a dense array of source, drain and gate connections in order to form field-effect devices. Subsequently, a voltage is applied over the nanotube ropes blowing out and destroying the metallic tubes, but leaving the semiconducting type unscathed. Thus, the surviving semiconducting nanotubes are available and still affixed as ropes to the contacts, where they may be utilized to produce active devices. However, the method provides no means of physically segregating or sorting the nanotubes into separate assemblies or containers. Nor does it provide a means for accumulating the highly conductive nanotubes as well as the metallic nanotubes.
Selective functionalization of metallic SWNTs has been disclosed, wherein selective functionalization occurs with individual nanotubes wrapped in surfactant molecules. However, separation of functionalized from the unfunctionalized nanotubes by selective solubility, sedimentation, or centrifugation has not yielded feasible separation processes. All of these rely heavily on the premise that stabilized nanotubes should remain in the supernatant while nonstabilized nanotubes should have significant aggregation allowing efficient separation. However, hydrodynamic instabilities will result in the contact of nonstabilized nanotubes (unfunctionalized) with stabilized nanotubes (functionalized) during sedimentation. This contact leads to both functionalized and unfunctionalized becoming trapped in irreversible aggregates as agglomeration is induced through sedimentation, limiting the effectiveness of the separations.
Chromatography may possibly be able to offer some separation of nanotubes by type but these suspensions are inherently instable, thereby affecting the absorption/desorption process critical to effect nanotube separations. Electrophoresis can be utilized to obtain a degree of nanotube separation while DNA-based chromatography techniques have also achieved a limited degree of nanotube separation. The major problem with these techniques is that they are only analytical-scale techniques and cannot produce large, significant quantities of nanotubes of a specific type.
While a method for separating and sorting single-wall carbon nanotubes of a specific type is desired in order to capture the desired properties of the selected nanotube type or types, such a method is complicated by two major factors. First is the nanotubes' extreme lack of dispersibility in water and most common solvents. Second, as described earlier, is the strong propensity of single-wall carbon nanotubes to “rope” together in bundles that are strongly held together by van der Waals forces. The roping phenomenon aggregates different types of single-wall carbon nanotubes together in aligned bundles or “ropes” and holds them together with a sizable tube-to-tube binding energy of up to about 500 eV/micron. These aggregates generally contain random mixtures of metallic and semiconducting types of nanotubes with assorted diameters. When electrically contacted while in bundled aggregates, the carbon nanotubes experience sizable perturbations from their otherwise pristine electronic structure that complicates the differentiation between different types of nanotubes. Also, attempts to exploit the chemical diversity within mixtures of nanotubes, either through sidewall functionalization or end-group derivatization have not been successful in separating nanotubes of specific conformations, but have produced largely bundles of nanotubes or nanotubes with significantly altered electronic properties.
No effective process for making single-wall carbon nanotubes is known whereby significant quantities of carbon nanotubes of a specific (n, m) type can be extracted after production/manufacture. Macroscopic amounts of type-sorted single-wall carbon nanotubes that would provide the broadest range of possible nanotube properties and applications are heretofore unknown.
The lack of any viable type separation of nanotubes has precluded their use in a multitude of commercial applications. The ability to separate single-walled carbon nanotubes by their type will be vital to a multitude of applications. The different types of nanotubes can be easily integrated into a wide variety of microelectronic devices, energy applications, and optical sensors. For example, metallic nanotubes can be constructed into quantum wires which will supply low energy-loss, high-throughput wires for energy savings and applications. Semiconductor nanotubes can be utilized in the formation of field-effect transistors in microelectronics or as implantable biosensors. Even the less ambitious goal of separating the metallic nanotubes from the semiconducting nanotubes will be a significant advance that will enable many new applications.